The transmission of data in the baseband using pulse amplitude modulation (PAM) is advantageous particularly when additional signals, such as voice signals for an additional telephone channel, do not need to be transmitted simultaneously in the audio frequency range. In contrast to carrier modulated transmission systems, such as QAM (quadrature amplitude modulation) or DMT (discrete multitone modulation) transmission systems, PAM transmission systems use virtually the whole frequency range starting at a bottom cut-off frequency, which is essentially determined by the characteristics of the line access circuit.
Pulse amplitude modulation is also used in duplex transmission systems, inter alia, in which data are transmitted simultaneously in both directions of the transmission channel or of the transmission line. Such duplex transmission systems require echo compensation in order to suppress the crosstalk from their own transmitter to the receiver in the same transmission unit, which would result in echo effects. The echo compensation simultaneously manages to make it possible to use the available bandwidth in optimum fashion at both ends, so that such transmission systems are distinguished, in particular, by a relatively long range for a prescribed interference environment.
FIG. 3 shows the basic arrangement of a PAM receiver in such a duplex data transmission system. A received signal u(t) is filtered by an analogue input filter 1 and is then sampled at the symbol rate 1/T by a sampler 2, so that these samples of the received signal are available at intervals of k·T. Instead of the analogue input filter 1, it is also possible to use a digital input filter if the sampling frequency is chosen to be appropriately high. Sampling at the symbol rate 1/T can be followed by a further filter stage 5, which is generally produced by a digital high-pass filter. This further filter 5 is used, in particular, to suppress low frequency interference, such as the offset, and to improve the transient response. An echo compensator 6 produces an echo compensation signal yec(k·T) on the basis of the transmitted data x(k·T) from the transmitter in the same duplex transmission unit and subtracts it from the sampled and equalized received signal y′(k·T) using the adder 7 shown in FIG. 3. The received signal echo-compensated in this way is finally equalized and is output as y(k·T) for further processing, in particular for demodulation, so that the respectively transmitted data can be recovered. The linear equalizer 8 used is generally a digital nonrecursive filter whose coefficients respectively need to be set adaptively to the current transmission channel. Since the received-signal values sampled at the symbol rate 1/T, filtered and freed of echo are supplied to the equalizer 8 as input signal, the equalizer 8 is also referred to as a T equalizer. Downstream of the equalizer 8, a decision feedback equalizer 9 is normally used in addition which compensates for the post-transients of the pulse response for the respective transmission channel and generally results in a better transmission response.
In many instances of application, a better transmission response can be attained for the same interference environment if an equalizer is used whose input signal is sampled at twice the symbol rate of the received signal, i.e. at the frequency 2/T. Such an equalizer is therefore also referred to as a T/2 equalizer.
A corresponding receiver arrangement having such a T/2 equalizer is shown in FIG. 4, where those elements which correspond to the elements shown in FIG. 3 have been provided with the same reference symbols. As can be seen in FIG. 4, the received signal u(t) is sampled at twice the symbol rate 2/T by the sampler 2 and is supplied to the echo compensator 6 via the digital high-pass filter 5. On account of the sampling frequency being doubled, the echo compensator has to produce two compensation values y(k·T/2) per received symbol in this case. The received signal echo-compensated in this way is supplied to the T/2 equalizer 8 and is sampled at the output of the T/2 equalizer at once the symbol rate 1/T by a further sampler 13 and is output to the decision feedback equalizer 9.
The fundamental drawback of this receiver arrangement is that the echo compensator 6, as has already been explained, has to produce two compensation values per received symbol, i.e. twice as many compensation values as in the case of the arrangement shown in FIG. 3. For this reason, the complexity of producing the echo compensator 6, which is the main portion of the total complexity anyway, is virtually doubled.
This is intended to be demonstrated by the illustration shown in FIG. 5, which shows a possible circuit arrangement for the echo compensator 6 shown in FIG. 4 for a transmission system having a T/2 equalizer 8. The echo compensator 6 essentially comprises two paths, with the upper path generating the components of the echo compensation signal yec(k·T) for the sampling instants k·T+T/2, and the lower path generating the components of the echo compensation signal for the sampling instants k·T. The compensation values generated by the two paths using delay elements 14, multipliers 15 with settable multiplication coefficients h1,1 . . . hN,1 and h1,2 . . . hN,2, and adders 16 are forwarded alternately at the output. An echo compensator for a transmission system having a T equalizer would, by contrast, require only one path, since in that case only one compensation value would need to be generated per received symbol.
DE-C-211 029 discloses a generic reception method for duplex transmission and a generic associated receiver arrangement which sample a received signal at twice the symbol rate of the received signal before echo compensation and equalization.
DE 30 09 450 A1 discloses an echo cancellation arrangement for homochronous data transmission systems, where the received signal is also sampled at once the symbol rate, but only after echo compensation.
“Adaptive Sprecherecho-Kompensation in Modems fur die Duplex-Datenubertragung im Fernsprechnetz” [Adaptive Speaker Echo Compensation in modems for duplex data transmission in the telephone network], Frequenz 6/1983, pp. 145-153, likewise discloses respective sampling before and after the echo compensation.
DE 38 28 623 C2 discloses a method for producing phase shifts for phase modulation or phase keying or quadrature amplitude modulation.